In recent years, Blu-ray Disc (BD) have received a lot of attention as a large-capacity recording medium. In comparison with digital versatile disc (DVD), single layer BD has a recording density approximately five times higher, and dual layer BD has the one approximately ten times higher. However, multi layer BD has been expected as a larger-capacity recording medium so that various kinds of large-volume information are enjoyed. Furthermore, with improved data transmission rate, an amplifier circuit which supports higher-speed recording and play mode has also been expected.
FIG. 14 is a circuit configuration diagram illustrating a conventional photodiode amplifier circuit. As illustrated in FIG. 14, in circuit for mixing four-channel signals into an RF signal, 501 to 505 denote operational amplifiers, 511 to 514 denote photodiodes, 521 to 524 denote feedback resistors, 531 to 535 denote resistors, 541 to 545 denote output terminals, and 546 and 547 denote reference voltage sources.
The photodiode amplifier circuit is described in details. A laser light reflected from an optical disc is incident on the photodiode 511, and an electric current of an amount corresponding to the amount of the incident light pass through the photodiode 511. The operational amplifier 501 and the feedback resistor 521 are included in a negative feedback circuit. The electric current generated in the photodiode 511 is converted into a voltage through the feedback resistor 521, and then outputted from the output terminal 541 as a voltage signal. Similarly, the electric currents generated in the photodiodes 512 to 514 are converted into voltages through the feedback resistors 522 to 524, and then outputted from the output terminals 542 to 544 as voltage signals, respectively. The currents of the signals from the output terminal 541 to 544 are added up into one signal through the resistors 531 to 534. The signal is amplified in the resistor 535 and the operational amplifier 505, and then outputted to the output terminal 545 as an RF signal.
However, an optical disc suffers from a lower reflectivity if the disc is a multi-layered one or if the disc is played in hi-speed play mode. The amount of the laser light incident on the photodiodes 511 to 514 becomes smaller, thereby reducing signal quality. As a result, the information recorded on the disc cannot be read. In order to solve this problem, the amplification factor of the amplifier circuit has to be larger by increasing the resistance values of the feedback resistors 521 to 524. However, the resistance values of the feedback resistors 521 to 524 are increased, the thermal noise generated in the feedback resistors 521 to 524 is increased, thereby worsening the signal-to-noise ratio in the result. On the other hand, each of the photodiodes 511 to 514 has a parasitic capacitance. The capacitance value of each parasitic capacitance is determined by the size of each of the photodiodes 511 to 514. The sizes of the photodiodes 511 to 514 are determined by the components of the optical system, so there is a natural limit to the capacitance values of the photodiodes 511 to 514. As a result, in the pole frequency, which is determined by the feedback resistors 521 to 524 and the capacitance values of the photodiodes 511 to 514, the frequency band is limited. This means that further higher-speed play mode becomes difficult. Blu-ray Discs are facing a problem that the improvements in signal-to-noise ratio and speeding up are not achieved at the same time.
In order to solve these problems, Japanese Patent Publication No. 2002-92882 has proposed a photodiode amplifier circuit in which an avalanche photodiode (hereinafter, referred to as APD) employed for photodiode, and Japanese Patent Publication No. 2008-234811 has proposed a photodiode amplifier circuit in which one ends of the photodiode terminals are connected to a common node to generate an RF signal. FIG. 15 is a circuit configuration diagram illustrating a conventional photodiode amplifier circuit in which an avalanche photodiode according to Japanese Patent Publication No. 2002-92882 is employed. FIG. 16 is a circuit configuration diagram illustrating a photodiode amplifier circuit according to Japanese Patent Publication No. 2008-234811, in which one-end sides of photodetectors are connected to a common node to generate an RF signal.
In FIG. 15, 601 denotes a laser light source, 602 denotes a collimator lens, 603 denotes a polarizing beam splitter, 604 denotes a quarter-wave plate, 605 denotes an object lens, 606 denotes an optical disc, 607 denotes a beam splitter, 608, 609 and 612 denote photodetectors, 610 denotes a detecting lens, 611 denotes a cylindrical lens, 613 denotes a comparator, 614 denotes a reverse-bias-voltage control circuit, and 615 denotes a fixed-voltage selector.
The photodiode amplifier circuit of FIG. 15 is described in details. A laser light emitted from the laser light source 601 is converted from divergent light to parallel light by the collimator lens 602. The parallel laser light is split by the polarizing beam splitter 603 into the laser light directed to the optical disc 606 and the laser light directed to the photodetector 608. After passing through the quarter-wave plate 604, the laser light directed to the optical disc 606 is concentrated with the object lens 605, and applied onto the optical disc 606. The applied laser light is reflected from the optical disc 606. The reflected laser light is directed to the photodetector 612 through the object lens 605, the quarter-wave plate 604, and the polarizing beam splitter 603. The laser light directed to the photodetector 612 is incident on the photodetector 612 through the detecting lens 610 and the cylindrical lens 611. The photodetector 612 detects the signal information of the optical disc 606 according to laser light intensity. Furthermore, the laser light directed toward the photodetector 608 is split by the beam splitter 607 into the laser light directed to the photodetector 608 and the laser light directed to the photodetector 609. The photodetector 608 detects the light amount of the laser light source 601, and gives the detection result to the laser light source 601 as feedback, so that the laser light source 601 has a constant light amount. On the other hand, the laser light directed to the photodetector 609 is incident on the photodetector 609, and the photodetector 609 detects the voltage corresponding to the light amount of the laser light source 601. The comparator 613 makes a comparison between the voltage value detected by the photodetector 609 and the voltage value selected by the fixed-voltage selector 615. Based on the comparison result of the comparator 613, the reverse-bias-voltage control circuit 614 generates a reverse bias voltage. The generated reverse bias voltage is applied to the photodetectors 609 and 612, thereby controlling the reverse bias voltages of the photodetectors 609 and 612. This configuration achieves a highly sensitive but stable amplifier which can resist environmental changes, such as temperature.
Next, the photodiode amplifier circuit of FIG. 16 is described. In FIG. 16, 701 to 705 denote operational amplifiers, 711 to 714 denote photodiodes, 721 to 724 and 735 denote feedback resistors, 741 to 745 denote output terminals, 746 and 747 denote reference voltage sources, 751 denotes an inductor, and 752 and 753 denote capacitances.
The operation of the photodiode amplifier circuit of FIG. 16 is hereinafter described. A laser light is incident on the photodiode 711, and the photodiode 711 then generates the electric current corresponding to the incident light amount. The operational amplifier 701 and the feedback resistor 721 are included in a negative feedback circuit. As the electric current is generated in the photodiode 711, the electric current passing through one end thereof is converted into a voltage through the feedback resistor 721 and the operational amplifier 701, and then outputted from the output terminal 741 as a signal. Similarly, the electric currents generated in the photodiodes 712 to 714 are converted into voltages through the feedback resistors 722 to 724, and then outputted from the output terminals 742 to 744 as signal, respectively. The electric currents generated in the photodiodes 711 to 714 passing from the other ends thereof are added up into one electric current signal. The electric current signal is inputted to an amplifier including the operational amplifier 705 and the feedback resistor 735 through the inductor 751. The inputted electric current signal is converted into a voltage signal by the feedback resistor 735 and the operational amplifier 705, and then outputted from the RF output terminal 745 as an RF signal.
As has been described above, the photodiode amplifier circuit according to Japanese Patent Publication No. 2002-92882 employs an APD as a photodiode of each photodetector. Therefore, the photodiode amplifier circuit according to Japanese Patent Publication No. 2002-92882 has an optical sensitivity several times to dozen times greater than the case in which a common PIN photodiode is employed therefor. Specifically, as the optical sensitivity becomes higher, the signal becomes larger in amplitude. Therefore, the ratio of signal to noise becomes larger, with the result that the signal-to-noise ratio is improved, thereby further improving signal quality.
In the photodiode amplifier according to Japanese Patent Publication No. 2008-234811, the four-channel photodiodes share one anode, so four-channel electric currents generated in the respective photodiodes are added up into one signal. The signal is converted into an RF signal through the feedback resistors. Therefore, the thermal noise becomes one-fourth and the noise level of the RF terminal 745 is reduced by 6 dB, in comparison with the case in which a single photodiode is employed. Therefore, the ratio of noise to signal becomes smaller, with the result that the signal-to-noise ratio is improved, thereby further stabilizing signal quality.